On development of hypoeutectic aluminium–silicon powder metallurgy alloy

Abstract

Powder metallurgy allows for the rapid, automated and efficient production of many different types of automotive components. However, a drawback is the limited selection of readily available light alloy blends. Owing to the wide spread use of aluminium–silicon casting alloys for existing components it is logical to develop aluminium–silicon PM options. Therefore, an experimental hypoeutectic aluminium–silicon alloy was chosen for study and an optimum processing route developed. Tests were performed to determine the green strength and density as a function of compaction pressure. Sintering conditions were optimised based on sintered density, hardness and dimensional changes. Metallography, differential scanning calorimetry and energy dispersive X-ray spectroscopy analysis provided insight into post-sinter furnace cooling and heat treatment parameters. An appropriate T6 heat treatment was developed and samples were tested in tension. The alloy was able to achieve a high sintered density approaching 98% and a yield strength of 232 MPa under the T6 condition.     

On hot deformation of aluminium–silicon powder metallurgy alloys

Abstract

The growing field of aluminium powder metallurgy (PM) brings promise to an economical and environmental demand for the production of high strength, light weight aluminium engine components. In an effort to further enhance the mechanical properties of these alloys, the effects of hot upset forging sintered compacts were studied. This article details findings on the hot compression response of these alloys, modelling of this flow behaviour, and its effects on final density and microstructure. Two aluminium–silicon based PM alloys were used for comparison. One alloy was a hypereutectic blend known as Alumix-231 (Al–15Si–2·5Cu–0·5Mg) and the second was an experimental hypoeutectic system (Al–6Si–4·5Cu–0·5Mg). Using a Gleeble 1500D thermomechanical simulator, sintered cylinders of the alloys were upset forged at various temperatures and strain rates, and the resulting stress–strain trends were studied. The constitutive equations of hot deformation were used to model peak flow stresses for each alloy when forged between 360 and 480°C, using strain rates of 0·005–5·0 s^?1. Both alloys benefited from hot deformation within the ranges studied. The experimental alloy achieved an average density of 99·6% (±0·2%) while the commercial alloy achieved 98·3% (±0·6%) of its theoretical density. It was found that the experimentally obtained peak flow stresses for each material studied could be very closely approximated using the semi-empirical Zener–Hollomon models.     

On development of press and sinter Al–Ni–Mg powder metallurgy alloys

Abstract

The objective of this research was to initiate the development of powder metallurgy alloys based on the Al–Ni–Mg system. In doing so, binary (Al–Mg) and ternary (Al–Ni–Mg) blends were prepared, compacted and sintered using elemental and master alloy feedstock powders. Research began with fundamental studies on the sintering response of the base aluminium powder with additions of magnesium. This element proved essential to the development of a well sintered microstructure while promoting the formation of a small nodular phase that appeared to be AlN. In Al–Ni–Mg systems a well sintered structure comprised of α aluminium plus NiAl₃ was produced at the higher sintering temperatures investigated. Of these ternary alloys studied, Al–15Ni–1Mg exhibited mechanical properties that were comparable with existing commercial 'press and sinter' alloys. The processing, reaction sintering and tensile properties of this alloy were also found to be reproducible in an industrial production environment.     

Microstructure evolution and densification behaviour of powder metallurgy Al–Cu–Mg–Si alloy

ABSTRACT

An Al–Cu–Mg–Si alloy was prepared by conventional press-sintering powder metallurgy using elemental Al powder. The phase transformation process of Al–Mg, Al–Si alloy and Cu during the sintering process was investigated in details. It was found that a series of phase transitions take place in the alloy to disrupt the oxide film of Al particle and enhance the densification process. The relative density of the sintered samples reached 98%. A new Al–Mg–Cu–O compound was found at the grain boundaries except the MgAl₂O₄ phase, it is speculated that the disruption of the oxide film was also associated with the other alloy compositions except for Mg. Furthermore, no detectable AlN compound was found at the grain boundary region although sintering with flowing nitrogen atmosphere, which is benefit from the high density of the green compact and the excellent wettability between the liquid phase and the aluminium.     

Effects of Fe and Ni additions on emerging Al–4·5Cu–1·5Mg powder metallurgy alloy

Abstract

In recent laboratory studies, a method designed to modify an emerging Al–Cu–Mg powder metallurgy alloy with Fe and/or Ni additions was successfully developed. The objectives of this research were designed to expand upon this work with an emphasis on characterising the industrial processing behaviour and thermal stability of the alloy with and without additions of these transition metals. All powder compacts exhibited an industrial sintering response that mirrored prior laboratory findings thereby confirming commercial viability. Subsequently, tensile specimens were machined from industrially sintered bars, heat treated to the T6 temper and then subjected to various conditions of thermal exposure at temperatures up to 280°C. Differential scanning calorimetry (DSC) data confirmed that Fe/Ni additions reduced the precipitation kinetics of the S-type phases responsible for peak strengthening at temperatures <160°C and thereby enhanced the thermal stability of important tensile properties such as yield strength. At higher temperatures (>200°C), the Fe/Ni additions were less effective with both alloys exhibiting comparable levels of strength degradation.

Lors d’études récentes en laboratoire, on a développé avec succès une méthode conçue pour modifier un alliage émergeant de la métallurgie des poudres, Al–Cu–Mg, avec des additions de Fe et/ou de Ni. Cette recherche a pour but d’élargir ce travail avec une emphase sur la caractérisation du comportement du traitement industriel et de la stabilité thermique de l’alliage, avec ou sans additions de ces métaux de transition. Tous les compacts de poudre exhibaient une réponse industrielle de frittage qui reflétait les trouvailles antécédentes en laboratoire, confirmant ainsi la viabilité commerciale. Subséquemment, on a usiné des échantillons pour mesure de résistance à la traction à partir de barres frittées industriellement et on les a traités thermiquement à l’état T6. On a ensuite soumis les échantillons à des conditions variées d’exposition thermique à des températures allant jusqu’à 280°C. Les données de DSC ont confirmé que les additions de Fe/Ni réduisaient la cinétique de précipitation des phases de type S, responsables de l’endurcissement de pointe à des températures <160°C et augmentaient ainsi la stabilité thermique des propriétés importantes de résistance à la traction comme la limite d’élasticité. À des températures plus élevées (>200°C), les additions de Fe/Ni étaient moins efficaces, les deux alliages exhibant des niveaux comparables de dégradation de la résistance.     

Microstructural characterisation of Ti–Nb–(Fe–Cr) alloys obtained by powder metallurgy

Abstract

β alloys based on the Ti–Nb alloy system are of growing interest to the biomaterial community. The addition of small amounts of Fe and Cr further increases β-phase stability, improving the properties of Ti–Nb alloy. However, PM materials sintered from elemental powders are inhomogeneous due to restricted solid state diffusion and mechanical alloying provides a route to enhance mixing and elemental diffusion. The microstructural characteristics and bend strength of Ti–Nb–(Fe–Cr) alloys obtained from elemental powder mixture and mechanical alloyed powders are compared. Mechanical alloying gives more homogeneous compositions and particle morphology, characterised by rounded, significantly enlarged particles. In the sintered samples α and β phase are observed. The α phase appears at the grain boundaries and in lamellae growing inward from the edge, and is depleted in Nb. The β phase is enriched with Nb, Fe and Cr. The addition of Fe and Cr significantly increases the mechanical properties of Ti–Nb alloys, providing increased ductility.     

Isothermal oxidation behaviour of beta gamma powder metallurgy TiAl–4Nb–3Mn alloys

Abstract

A new TiAl–4Nb–3Mn beta gamma alloy was synthesised by a powder metallurgy process. HIPed and vacuum heat treated specimens were isothermally oxidised at 800 and 900°C in air up to 500 h. The TiAl–4Nb–3Mn alloys oxidised parabolically up to 500 h at both 800 and 900°C. The oxides consisted of outer TiO₂ layer, intermediate Al₂O₃ layer and inner TiO₂ rich mixed layer and the oxidation mechanisms of the alloy were identical at both temperatures. During oxidation, the degradation of α₂ lamellae in the vicinity of the interface forms a diffusion zone (lamellar depleted zone) leading to the formation of Nb and Mn rich white layers just below the interface by outward diffusion of Nb and Mn which are released from breakdown of α₂ lamellae. As exposure time increases, Nb begins to diffuse earlier than Mn and diffuses more actively at higher temperature. The activation energy for oxidation of TiAl–4Nb–3Mn alloy was lower than that of Ti–48Al alloy and was higher than those of Ti–48Al–2Nb–2Cr and Ti–48Al–2Nb–2Cr–W alloys.

On a synthétisé un nouvel alliage TiAl–4Nb–3Mn de type bêta gamma par un procédé de métallurgie des poudres. On a oxydé en isotherme à 800 et à 900°C à l’air, jusqu’à une durée de 500 heures, les spécimens pressés par HIP et traités thermiquement sous vide. Les alliages de TiAl-4Nb-3Mn s’oxydaient paraboliquement jusqu’à 500 heures tant à 800°C qu’à 900°C. Les oxydes consistaient en une couche externe de TiO₂, en une couche intermédiaire d’Al₂O₃ et en une couche interne mixe riche en TiO₂ et les mécanismes d’oxydations de l’alliage étaient identiques aux deux températures. Lors de l’oxydation, la dégradation de lamelles d’α₂ dans le voisinage de l’interface forme une zone de diffusion (zone lamellaire appauvrie) menant à la formation de couches blanches riches en Nb et en Mn juste au-dessous de l’interface par diffusion vers l’extérieur du Nb et du Mn, qui sont relâchés par la dégradation des lamelles d’α₂. À mesure que la durée de l’exposition augmente, le Nb commence à se diffuser plus tôt que le Mn et se diffuse plus activement à une température plus élevée. L’énergie d’activation pour l’oxydation de l’alliage de TiAl–4Nb–3Mn était plus basse que celle de l’alliage de Ti–48Al et était plus élevée que celle des alliages de Ti–48Al–2Nb–2Cr et de Ti–48Al–2Nb–2Cr–W.     

Iron–manganese: new class of metallic degradable biomaterials prepared by powder metallurgy

Abstract

An Fe–35 wt-%Mn alloy, aimed to be used as a metallic degradable biomaterial for stent applications, was prepared via a powder metallurgy route. The effects of processing conditions on the microstructure, mechanical properties, magnetic susceptibility and corrosion behaviour were investigated and the results were compared to those of the SS316L alloy, a gold standard for stent applications. The Fe35Mn alloy was found to be essentially austenitic with fine MnO particles aligned along the rolling direction. The alloy is ductile with a strength approaching that of wrought SS316L. It exhibits antiferromagnetic behaviour and its magnetic susceptibility is not altered by plastic deformation, providing an excellent MRI compatibility. Its corrosion rate was evaluated in a modified Hank's solution, and found superior to that of pure iron (slow in vivo degradation rate). In conclusion, the mechanical, magnetic and corrosion characteristics of the Fe35Mn alloy are considered suitable for further development of a new class of degradable metallic biomaterials.     

Effects of Fe and Ni additions on an emerging Al–4·4Cu–1·5Mg powder metallurgy alloy:

Abstract

The effects of Fe and Ni additions to an Al–Cu–Mg powder metallurgy alloy were studied. The transition elements were incorporated through two different approaches, admixed elemental powders and prealloying of the base Al powder. Prealloying proved to be the superior alloying technique as it did not invoke any negative effects on the general sintering behaviour of the baseline alloy. The microstructures of prealloyed materials also exhibited a refined distribution of the aluminide phases formed. These included Al₇Cu₂Fe, Al₇Cu₄Ni and Al₉FeNi as identified through a combination of SEM/EDS and XRD. The most promising alloy was identified as Al–4·4Cu–1·5Mg–1Fe–1Ni as it exhibited tensile properties that exceeded those of the baseline material. This advantage was also found to persevere after thermal exposure.

On a étudié les effets d’additions de Fe et de Ni sur un alliage d’Al–Cu–Mg obtenu par la métallurgie des poudres. On a incorporé les éléments de transition au moyen de deux approches différentes – poudres élémentaires ajoutées ou préalliage de la poudre d’Al de base. Le préalliage était supérieur comme technique d’alliage puisqu’il n’invoquait pas d’effets négatifs sur le comportement général de frittage de l’alliage de référence. La microstructure des matériaux de préalliage exhibait également une distribution raffinée des phases d’aluminures formées. Celles-ci incluaient Al₇Cu₂Fe, Al₇Cu₄Ni et Al₉FeNi, telles qu’identifiées au moyen d’une combinaison de SEM/EDS et de XRD. L’alliage le plus promettant était l’Al–4·4Cu–1·5Mg–1Fe–1Ni, puisqu’il exhibait des propriétés de traction qui excédaient celles du matériau de référence. On a également trouvé que cet avantage persistait après une exposition thermique.     

Friction and wear behaviours of surface densified powder metallurgy Fe–2Cu–0.6C material

Surface rolling was employed to fabricate a densified layer on a powder metallurgy (PM) Fe–2Cu–0.6C piece. A densified surface layer with a depth of 335?μm and a surface hardness of 330?HV_0.1 was obtained, in which the lamellar spacing of pearlite and grain size of ferrite were refined. Friction and wear behaviours of the surface densified material were studied. Results indicated that friction coefficient of the rolled material decreased as the load increased, which was lower than that of the un-rolled material. Wear volumes were lower than that of the un-rolled material, which increased as the load increased. Wear loss was caused by flake spalling and grooves, and the wear mechanism mainly was abrasive wear. The surface densified layer with higher hardness and lower porosity can hinder the cracks initiation and propagation on the surface and under the surface, which enhance the wear resistance of the PM material.     

Tribological properties of the babbit B83–based composite materials fabricated by powder metallurgy

Technological processes are developed to fabricate composite materials based on B83 babbit using hot pressing of a mixture of powders in the presence of a liquid phase. As a result, the structure of the matrix B83 alloy is dispersed, the morphology of intermetallic phases is changed, and reinforcing micro- and nanosized fillers are introduced and uniformly distributed in the matrix. The tribological properties of the synthesized materials are studied. The friction of the B83 babbit + 0.5 wt % MSR + 3 wt % SiC (MSR is modified schungite rock) composite material at high loads is characterized by an increase in the stability coefficient, and the wear resistance of the material increases by a factor of 1.8 as compared to the as-cast alloy at comparable friction coefficients.     

Effects of trace ytterbium addition on microstructure,mechanical and thermal properties of hypoeutectic Al–5Ni alloy

Over the past decade, the cast aluminum alloys with excellent mechanical and conductivity properties have emerged as potential materials for thermal management. However, the traditional Al–Si based alloys are difficult to make significant breakthrough in conductivity performance. The hypoeutectic Al–5Ni alloy also possesses sound castability and is expected to be applied in thermal management applications. In this study, the effects of ytterbium (Yb element) at 0–0.5 wt% on the microstructures as well as the electrical/thermal conductivity and mechanical properties of the Al–5Ni alloy were systematically investigated. The experimental results indicate that the addition of Yb at a relatively low amount not only reduces the secondary dendrite arm spacing of the α-Al grains, but also modifies the morphology and distribution of eutectic boundary phase. Moreover, it is found that the dosage of Yb at 0.3 wt% in the Al–5Ni alloy can simultaneously improve the yield strength, ultimate tensile strength and electrical/thermal conductivity. The strengthening and toughening of the Al–5Ni alloy are mainly attributed to the decrease of secondary dendrite arm spacing and the improvement of eutectic phases. The transmission electron microscopy/selected area electron diffraction (TEM/SAED) analysis indicates that the ytterbium in Al–5Ni alloy will form Al₃Yb phase, which mainly agglomerates in the Al₃Ni phase region. This phase is helpful to decrease the solubility of impurity elements (e.g., Fe and Si) in the α-Al matrix, which is beneficial to electrical/thermal conductivity. The value of this study lays foundation for manufacturing Al–Ni alloys with high thermal conductivity and acceptable mechanical properties.     

Synthesis of pewter alloy from tin–copper–antimony powder mixtures by microwave and conventional sintering

Abstract

Two compositions of pewter alloy were sintered using both microwave and conventional vacuum sintering, and the effects of sintering time, temperature and weight percentage of copper and antimony on the mechanical and structural properties were examined for both sintering methods. Microwave sintered samples had finer microstructures, higher densities, higher hardness and tensile strength compared to the conventionally sintered samples and traditionally cast pewter. By increasing the copper and antimony contents, higher hardness was achieved. Better mechanical properties were found after microwave sintering after shorter sintering times compared with conventional sintering, but longer sintering times resulted in better diffusion for both sintering methods. The microwave sintered samples in general were capable of achieving similar amounts of diffusion to those conventionally sintered for the same time. But the total sintering process is much faster in microwave heating than in conventional heating due to the rapid heating effect.     

Rotary cold re-pressing and heat treatment of sintered materials from Co–Cr–Mo alloy powder

Abstract

This paper presents the analysis of research results concerning the application of PM technology to produce porous implantation material from Co-Cr-Mo alloy. Rotary cold re-pressing and heat treatment has been used to increase the density and mechanical properties of sintered samples. The microstructure, hardness and compressive properties, and ultrasonic data of the obtained materials were investigated. The material had about 10% of porosity and has higher mechanical properties, i.e. UCS and plastic strain, compared with cast cobalt alloy, and had 25% lower values of Young's modulus and shear modulus.     

The effects of Zr and Ti on the microstructure and tensile properties of Al–Cu–Mg aluminium alloy

The effects of Al–5Ti–1B and Al–15Zr master alloys on the structural characteristics and tensile properties of Al–4.5Cu–0.3Mg aluminium alloy were studied. Al–5Ti–1B was found to be more effective than Al–15Zr in grain refining of the alloy. It could be seen that average grain size reduces from 570 to 260?μm when 0.01?wt-% Ti addition; additionally, while different amounts of Ti and Zr were added to the alloy, the dendritic structure changes from long dendrite to rather rosette-like morphology. Furthermore, tensile testing of cast specimens revealed that ultimate tensile strength (UTS) of the cast alloy increases from 241 to 283 and 260?MPa after adding the optimum amount of Ti and Zr containing master alloys, respectively. Moreover, UTS values of T6 heat-treated specimens also showed 73 and 61% improvement after adding 0.05?wt-% Ti and 0.3?wt-% Zr to the alloy. Fracture surface examinations exhibited a transition from brittle fracture mode in as-cast to ductile fracture in refined and T6 heat-treated specimens.     

Microstructural investigation of controlled short circuiting gas metal arc welding deposited aluminium–lithium alloy

Controlled short circuiting gas metal arc welding (CSC-GMAW) was investigated as a potential solid freeform fabrication (SFF) process for AA2199. The low heat input of the CSC-GMAW process resulted in a cooling rate on the order of 840–3500°C s^?1 being realised during deposition. The solidification time was then calculated to range between 2–5 ms depending on the location within the weldment. The deposited material displayed a fine (4·3±1 μm) cellular structure, comparable to that previously reported for electron beam welding. Through comparison with the Kurz–Giovanola–Trivedi (KGT) model for microstructural development during solidification, the solidification front velocity (SFV) of the CSC-GMAW process was estimated to be ～2–4·5×10^?4 m s^?1. Chemical analysis revealed lateral segregation of copper to the cell walls. TOF-SIMS revealed a homogeneous lateral lithium distribution, however depth profiling displayed some extent of lithium enrichment at the surface of the deposited material.

On a examiné le soudage à l’arc sous protection gazeuse à court-circuit contrôlé (CSC-GMAW) comme processus potentiel de fabrication en forme libre solide (SFF) pour l’AA2199. Le faible débit de chaleur du processus de CSC-GMAW produisait une vitesse de refroidissement de l’ordre de 840 à 3500°C/s lors du dépôt. On a ensuite calculé que le temps de solidification variait de 2 à 5 ms, dépendant de l’emplacement dans la soudure. Le dépôt montrait une structure cellulaire fine (4·3±1 μm), comparable à la structure rapportée antérieurement pour le soudage par faisceau d’électrons. Par comparaison avec le modèle de Kurz–Giavanola–Trivedi (KGT) de développement de la microstructure lors de la solidification, on a estimé que la vélocité du front de solidification (SFV) du processus de CSC-GMAW était de ～2 à 4·5×10^?4 m s^?1. L’analyse chimique a révélé une ségrégation latérale du cuivre sur les parois de la cellule. TOF-SIMS a révélé une distribution latérale homogène du lithium, cependant le profilage en profondeur a exposé une certaine zone d’enrichissement en lithium à la surface du matériel déposé.     

Effect of composition and milling time on mechanical and wear performance of copper–graphite composites processed by powder metallurgy route

Abstract

Copper–graphite (Cu–Gr) composites with 0, 5, 10 and 15 vol.-% graphite were processed via powder metallurgy route. The effect of composition and milling time on mechanical properties and wear resistance were studied. With increase in vol.-% of graphite, there was decrease in hardness of the composites. However, increasing milling time showed significant increase in hardness of the composites. Compressive strength of the composites containing 5 and 10 vol.-% of graphite was found to be 515 and 393 MPa respectively. The wear tests were carried out using a block-on-ring tribometer at a load of 30 N with varying sliding speed. The wear performance of the composites was found to be better with increase in milling time. The worn surfaces were analysed using FESEM. With increase in graphite content from 5 to 15 vol.-%, the coefficient of thermal expansion of the Cu–Gr composites decreased from 14·1 to 12·2×10^?6/°C.     

Effects of content and particle size of Si crystal on damping property of powder forged Al–Si alloy

Abstract

The effects of the Si content and Si crystalline size on the damping properties of powder forged (PF) Al–Si alloys has been investigated using a fast Fourier transformation (FFT) analyser. A quantitative image analysis method was used successfully to determine the mean diameter and distribution of Si crystals in the aluminium alloy. The damping capacity greatly depends on the mean diameter of Si crystals. The value of δ for heat treated specimens with Si crystals of 2–5 μm is 4 to 25 times less than that for the as forged specimens with Si crystals of 1–5 μm. This indicates that the vibration is absorbed at the interface between the Si particles and the aluminium matrix by a mechanism of internal friction. Therefore, a fine distribution of Si crystals and/or elimination of crystal coarsening should contribute to improved damping.     

Microstructure and mechanical properties of SiC particles reinforced Mg–8Al–1Sn magnesium matrix composites fabricated by powder metallurgy

The new type of Mg–8Al–1Sn (AT81) magnesium matrix composites reinforced with different volume fractions (5, 10, 15, 20, 25 and 30 vol.-%) of SiC particles (average size of 10 μm) was fabricated by powder metallurgy. With the increasing volume fraction of SiC particles (SiC_p), the particles gradually show more homogeneous distribution. Compared with the AT81 alloy, the yield strength (YS) and ultimate compressive strength of the SiC_p/AT81 composites are improved simultaneously. With the increasing SiC_p from 0 to 30 vol.-%, the YS and ultimate compressive strength increase from 69 to 239 MPa and 286 to 385 MPa respectively, while the corresponding fracture strain (ε) decreases from 19·3 to 4·8%. The improvement of the YS and ultimate compressive strength of the SiC_p/AT81 composites benefits from the more homogeneous microstructure due to the increase in the SiC particles.